High-volume injection molding machines can be either categorized by the type of injection unit or clamping unit regardless of the molding material. There are two main categories of injection units are the ram type and the screw type and two main categories of clamping units are the hydraulic type and the mechanical type. The source of the driving unit can be either hydraulic or electrical. The stringent control of the hold pressure of the injection process is the key element to controlling the final quality of the plastic molds. Common problems such as surface wrinkles, warpage, distortion, flash, bleeding, air holes, and short fill are usually caused the deficient fill or hold pressure during the injection process. FIGS. 4A to 4E consecutively illustrate the injection process of a hydraulic screw type injection molding machine for the steps of locking the mold, injection, holding the pressure, cooling, and releasing the mold. The relationship in the pressure variation and deviation in every step after the injection have direct effect on the quality of the sample dies, especially in FIGS. 4B and 4C for the steps of injection and holding the pressure where the switching pressure and switching position (the position of the injection axle) are of crucial importance. If the position is switched too early a short fill situation will occur, and oppositely if the position is switched too late a bleeding situation will occur. Similarly if the switching pressure is too high the plastic will have obvious parting lines, and oppositely if the switching pressure is unstable (the switching speed variation is too high) the plastic will experience peaky and unsmooth surfaces. Furthermore the switching position and the switching pressure also determine the value of the hold pressure in the subsequent step for compressing the resin to increase the density in FIG. 4C. If the hold pressure is too high the plastic melt will adhere to the wall and cannot release properly whose residual will further cause flash or bleeding and if the hold pressure is severely high the mold will be forced open. Oppositely if the hold pressure is too low the plastic will encounter warpage or have air holes which decreases the quality of the plastic. From the above description, it is to be understood that the injection switching is of high importance and yet the precise adjustment cannot be easily achieved. Please refer to FIGS. 5 and 5A for another example, the difficulty in controlling the hold pressure is illustrated, the Pm curve represents the pressure in the screw metering zone of the injection unit which is actually the default pressure curve. The Pn curve represents the pressure at the end of the nozzle, the Pg curve represents the pressure at the cavity entrance, and the Pc curve represents the pressure at the cavity exit. The Pc curve lies below the Pg curve due to the loss of pressure in the mold cavity. The periods from t1 to t3 indicate the injection step. During this time, the pressure in the cavity rapidly increases and the operator needs to perform the pressure control in this stage of the process. The periods from t3 to t4 indicate the pressure holding step. The hold pressure in this step is affected by the injection and switching pressure, position, and velocity from the previous step. The value of the hold pressure determines the effectiveness and performance of the pressure holding. The periods from t4 to t5 indicate the cooling step where the melt solidifies and the mold still experiences some remaining pressure.
From the aforementioned, the importance of the rapid pressure variation at different positions and the pressure holding during the injection molding pressure is emphasized. In general manufacturing processes, the control and switching parameter adjustment and control is entirely dependent on the skill of experience of the process engineer. The process engineer adjusts the parameters of the formula according to the quality of the molded die or examines the accuracy of the machinery in order to achieve consistency and stability in its manufacturing. However this trial and error method is difficult to control stability and a high level of repetition during high volume manufacturing. FIG. 6 shows a conventional hydraulic injection unit 30 (powered by a hydraulic motor 32). The operation involves determining the amount of travel by a potential meter or optical ruler during the injection process. The position of the injection axle 35 can be determined anytime during the injection process. When the position of the injection axle 35 reaches the predetermined V-P switching point, the injection unit 30 will automatically switch from the filling stage to the pressure holding stage. The control accuracy of this method is different every time because the position of the injection axle 35 is not identical every time. Due to lack of correction after each injection, the injection speed in the homestretch is deviated. Furthermore after repeated usage of the injection unit 30, the process parameter of the injection unit 30 will be altered (such as fluid quality, operating temperature, and the like) and consequently affects the accuracy and repetition of the control and decreases the quality of the molded die.
In newer electrical servo injection units, FIG. 7 shows the electrical servo motor 42 which is the driving source of the injection unit 40. An AC servo controller 43 and a servo motor 42 provide high accuracy control in position and velocity and high repetition consistency for completing the requirement for V-P switching and pressure holding. Other advantages such as reduction in cost, improvement in cleanliness, and improvement in operating quietness are realized. The V-P switching and pressure holding of the electrical injection unit 40 is more accurate than the hydraulic injection unit 30. Whereas the control method includes using a pressure control device to perform pressure close loop control for maintaining the injection filling pressure and hold pressure at a certain predetermined level for example in U.S. Pat. No. 5,154,935. In FIG. 8, a comparator compares the actual pressure to the operator-inputted target pressure curve in the control unit 51 and outputs the comparison result to servo loop 53 by two external switchable looping switches 52. The comparator therefore performs a closed-loop pressure control on the servomotor 55 that is coupled to the injection shaft 54 for maintaining a certain injection and hold pressure. Although the electrically-operated injection molding machines can eliminate the shortcomings of the hydraulic molding machines, they still possess problems such as manufacturing quantity because the device 50 can only perform closed-loop pressure control but cannot govern the injection speed and the injection shaft position. This method cannot effectively perform a smooth switching from the injection phase to the pressure holding phase because the selection of the switching position cannot be optimized and stabilized. Furthermore besides the pressure related characteristic parameter cannot be easily adjusted, the delay in the executing the switching order is significant because the switchable looping switches 52 of the conventional device 50 is designed to be external of the hardware.